Conventional optical fibers comprise a region of relatively high refractive index, the core, surrounded by a region of relatively low refractive index, the cladding. The cladding is typically surrounded by a polymer coating that generally plays no role in light guiding in transmission fibers.
There are also known "double clad" optical fibers that have a high-index core, surrounded by a region of intermediate refractive index, which in turn is surrounded by a region (typically polymer) of low refractive index that does play a role in light guiding.
Double clad fibers for instance find use in cladding pumped lasers. Such fibers typically have a silica-based single mode core doped with rare earth (e.g., Er and Yb). The region of intermediate refractive index typically is silica, possibly doped, of radius selected to make the structure a multimode waveguide at the pump wavelength. The low refractive index region could be air, but for practical reasons (e.g., fiber strength) generally is low refractive index polymer.
Pump light from low brightness sources, such as diode arrays, is easily coupled into the inner cladding of double clad fiber due to the inner cladding's large cross-sectional area and high numerical aperture (NA). As the multimode pump light crosses the core, it is absorbed by the rare-earth dopant. To increase the overlap of the pump light with the core, the inner cladding typically is made non-circular. The cladding pumped fiber can be made into a high-power, single-mode fiber laser by providing optical feedback at the appropriate wavelength in the core, typically by fiber Bragg gratings. Alternately, the fiber can be used as an amplifier by passing a signal through the single-mode core.
Cladding pumped lasers are used in the field of telecommunications to pump Raman lasers, Raman amplifiers, remotely located erbium amplifiers in repeaterless communication systems and high power Er/Yb amplifiers. In addition, these compact, solid-state, high-power, high-brightness lasers will undoubtedly find applications in many non-telecommunication fields such as material processing, printing and medical optics. One major advantage of cladding pumped fibers is their ability to convert low-brightness light from sources such as broad-stripe diode lasers into high-brightness light in a single-mode fiber.
Although cladding pumped fiber lasers and other devices that use dual clad optical fiber have advantageous properties, it would generally be desirable to be able to increase the amount of pump light that can be coupled into the fiber. This application discloses cladding pumped optical fiber having a structure that facilitates improved coupling of pump radiation into the fiber. A fiber of similar structure can provide long period gratings that have reduced temperature dependence.
Formation of long period gratings in optical fibers is a demanding process that typically comprises removal of the polymer coating from a length of optical fiber, "writing" of the grating into the fiber core while monitoring the grating properties, and re-coating the fiber. In this process it is frequently observed that re-coating of the fiber causes a change in the optical properties of the long period grating, due to the replacement of air (refractive index 1) with polymer (refractive index &gt;1) as the medium that surrounds the fiber cladding. Thus, it would be desirable to have available optical fiber that is re-coat insensitive, i.e., a fiber whose optical properties do not depend on the refractive index of the region immediately surrounding the outermost (preform-derived) glass cladding layer. This application also discloses optical fiber that is essentially re-coat insensitive.
U.S. Pat. No. 3,902,879 discloses a fiber with improved core-cladding interface, having elongate open channels extending longitudinally in the cladding.
U.S. Pat. No. Re 28,664 discloses a fiber having a transparent central portion attached by means of transparent thin film portions to the inside of a peripheral hollow cylindrical portion.
U.S. Pat. No. 3,712,705 discloses an air clad optical fiber comprising a dielectric core of polygonal cross section disposed within a circular dielectric jacket.
U.S. Pat. No. 4,046,537 discloses a fiber having a light conducting core supported within a protective sleeve by one or more thin support component.
U.S. Pat. No. 3,950,073 discloses an optical fiber comprising a glass core loosely disposed in an outer cylindrical glass jacket.